Organic semiconductor materials are currently being investigated for use as the active layer in a variety of devices including light-emitting diodes, nonlinear optical devices, and TFTs such as metal-insulator-semiconductor field effect transistors (MIS-FETs). Organic semiconductor materials offer processing advantages such as compatibility with flexible plastic substrates and ease of application by spin-coating and their use in processes for device fabrication is therefore attractive. However, in order for these materials to be useful for use in TFT devices, the resulting devices must have an on/off ratio (of the source/drain current) suited for the particular application. Although devices with on/off ratios as low a 100 are suited for some application, typically TFT devices must have an on/off ratio of at least about 10.sup.3. The properties of the organic semiconductor materials that govern the on/off ratio are carrier mobility and conductivity. Organic semiconductor materials typically have a carrier mobility in excess of about 10.sup.-8 cm.sup.2 /Vs but less than 1 cm.sup.2 /Vs. Consequently, based upon the relationship between carrier mobility, material conductivity, and device on/off ratio, the requisite conductivity of the organic semiconductor material is defined by the carrier mobility of the material and the desired on/off ratio.
A class of doped amorphous organic semiconductors is described in Brown, A. R., et al., "A universal relation between conductivity and field-effect mobility in doped amorphous organic semiconductors," Synthetic Materials, Vol. 68, pp. 65-70 (1994). Brown et al. report a linear relationship between the conductivity and the field effect mobility of such material, i.e., as the conductivity increases so does the field-effect mobility. Although Brown et al. report materials with a very high mobility, this high mobility was achieved at the expense of conductivity. Brown et al. conclude that high on/off ratios and high mobilities are not to be expected in devices constructed from amorphous organic semiconductors. Accordingly, if satisfactory devices are to be made from these materials, the field effect mobility and the conductivity of these materials must be within a range that provides a device with an on/off ratio of at least 10.sup.5.